1. Field of the Invention
This invention relates to a lighting device in which a discharge tube, such as an electrodeless tube, is caused to emit light by a high frequency electromagnetic field being applied to the discharge tube from the outside thereof.
In particular, it relates to a lighting device in which a discharge tube of an elongated shape can be quickly brought into a uniform light emitting state and which is suitable for the exposure of an original in an original reading apparatus.
2. Related Background Art
Fluorescent lamps and halogen lamps have heretofore been widely used in original reading apparatuses and everyday illumination.
A fluorescent lamp produces visible light and when viewed from the viewpoint of the wavelength of its emitted light, it permits the wavelength to be selected by selection of the fluorescent material and thus, it is preferable as an illuminating source, but if a great current is applied to its filament to obtain a great quantity of light, the filament is immediately burnt out and the quantity of light obtained is low. Also, when a current is caused to flow in the filament, the excited gas in the discharge tube accelerates the deterioration of the filament and thus, the service life of the filament itself is short.
As compared with a fluorescent lamp, a halogen lamp can provide a great quantity of light, but produces a great deal of light other than in the range of visible light, as shown in FIG. 28 of the accompanying drawings. That is, a halogen lamp produces a great deal of light which is not used in an apparatus utilizing chiefly the wavelength range of about 400-800 nm, such as an original reading apparatus or a copying apparatus having a photosensitive medium and therefore is low in power efficiency. Also, a halogen lamp produces light by converting electrical energy into heat and therefore suffers from great heat generation.
In view of such problems, Japanese Laid-Open Patent Applications Nos. 98457/1980 and 249240/1985 disclose applying energy to the discharge tube from the outside thereof by utilizing the discharge phenomenon as in a fluorescent lamp, and ensuring much higher brightness and much longer service life than a fluorescent lamp.
FIG. 27 of the accompanying drawings is a cross-sectional view of an example of such a light source. Reference numeral 64 designates a lamp having a fluorescent material 63 applied to the inner surface thereof and having mercury and inactivated gas enclosed therein. The lamp 64 is formed with a cylindrical portion 67 protruding so as to include a transformer 62. The transformer 62 comprises a core 66 and a coil 65, and the ends of the coil 65 wound around the core 66 are connected to a high frequency lamp source 61.
A high frequency voltage is applied from the high frequency lamp source 61 to the coil 65, whereby a high frequency electromagnetic field is produced around the coil 65. The electrical energy of this electromagnetic field excites the mercury gas in the lamp 64, and the ultraviolet rays of the mercury produced by this excitation are changed into visible light by the fluorescent material 63 applied to the surface of the lamp 64.
Such a light source utilizes the discharge phenomenon and can provide light of an appropriate wavelength range by the selection of the fluorescent material, and does not have any filament which emits heat electrons, and utilizes electromagnetic field energy applied by an electrode provided in contact with the outer wall of the discharge tube, and thus permits application of a great electric power thereto, is of high brightness and enjoys a long service life because the electrode is not exposed to the excited gas in the discharge tube.
Although such light source has merits of high brightness, long service life and good power efficiency because of its being appropriate to the wavelength range, it has suffered from the problem of a bad rising-up characteristic.
That is, even if high frequency power is supplied, much time is required before the lamp assumes a stable light-emitting condition, and this has led to the occurrence of the phenomenon that particularly in the worst case, the discharge does not occur over the entire discharge tube, but only partially. Such phenomenon is conspicuous where the discharge tube is of an elongated shape.